The invention relates to a method of electrophotographically manufacturing a display screen for a colour television display tube, comprising the steps of providing a firable, electrically conductive layer on the inside of the display window of the tube, providing on the layer a firable photoconductive layer, electrically charging the photoconductive layer, forming a latent charge image on the photoconductive layer developing the image into a pattern of charged particles from an apolar liquid, the charge on the particles being opposite to the charge with which the photo-conductive layer is charged and drying the pattern of particles.
Such a method is disclosed in U.S. Pat. No. 3,475,169, which also describes a few modified embodiments of the above electrophotographic method. For the formation of the charge image two exposure methods are possible. In accordance with the first method, a positive light image of the apertures in a colour selection electrode is formed. In accordance with the second method a negative light image is formed, which means that each aperture in the colour selection electrode is displayed by a shadow on the photoconductive layer. A suitable exposure method for the formation of a negative light image is the dark source method, in which use is made of two or more light sources at some distance from each other, as described in published German patent application No. 2,248,878 corresponding to British Patent Specification No. 1,396,918 published June 11, 1975.
The dispersions of charged phosphor particles and of charged, light-absorbing, particles in an electrically insulating liquid are, for example, those which are described in German patent application No. 19 28 817 laid open to public inspection, corresponding to British Patent Specification No. 1,318,396 published May 31, 1973. These dispersions consist of an apolar dispersion agent in which one or more surface-active, ion-forming substances are dissolved with such a dissociation capacity that their electric conductivity is larger than 10.sup.-12 Ohm.sup.-1 cm.sup.-1 (larger than 100 p S/m, preferably between 1.times.10.sup.-10 and 1.times.10.sup.-11 Ohm.sup.-1 cm.sup.-1 (between 10,000 and 1,000 p S/m) in which the phosphor particles in question are dispersed, At the phosphor particles-dispersion agent interface, the ion-forming substances build up a zeta potential. In practice the Siemens/meter has found acceptance in the last few years as a unit of electrical conductivity. The Siemens is equal to 1 Ohm.sup.-1, so that 1 p S/m is equal to 10.sup.-14 Ohm.sup.-1 cm.sup.-1.
In the method described, the photoconductive layer is, for example, charged to -300 V by means of a negative corona discharge. Those areas of the layer where no particles are to be provided are then exposed to light. For example, if a pattern of light-absorbing particles is to be provided before the phosphor patterns, those areas are exposed to light where the phosphor is to be later provided. This exposure is sufficient to form a latent charge image having the desired dimensions of the pattern. The charge image thus formed is developed with a suspension containing light-absorbing particles having a positive charge, which are deposited on the negatively charged areas between the exposed areas. The pattern of light-absorbing particles thus formed is then dried with compressed air. It has been found that the adhesion of particles provided in this manner is inadequate and damage to the pattern may occur.